Although state-of-the-art mechanical gyros and ring laser gyros are available for various applications, phase nulling fiber-optic gyros appear to have advantages over these types. These gyros have been disclosed in the article "Phase Nulling Fiber-Optic Laser Gyro", Optic Letters, Volume 4, page 93, March, 1979, by R. F. Cahill and E. Udd; "Solid-State Phase Nulling Optical Gyro" Applied Optics, Volume 19, page 3054, Sept. 15, 1980, by R. F. Cahill and E. Udd and "Techniques for Shot-Noise-Limited-Inertial Rotation Measurement Using A Multiturn Fiber Sagnac Interferometer" by J. L. Davis and S. Ezekiel published Dec. 13, 1978 in SPIE Volume 157, Laser Inertial Rotation Sensors as well as the above referenced Patents. The papers disclose early fiber-optic gyros which, although were shown to operate, had not yet been involved in the intensive development required to bring a concept to practical use. Problems inherent in ring laser gyros were bypassed by adopting passive cavity techniques and applying fiber optics. Nonlinear analog outputs of prior art fiber-optic gyros, which otherwise limit their dynamic range to a much lower level than that achieved by ring laser gyros are circumvented by using the phase nulling concept of Cahill and Udd.
The previously disclosed Cahill and Udd gyro is a linear rotation sensor rather than sinusoidal sensor. It produces an inherently digital output via a frequency change proportional to rotation rate. This gyro uses the nonreciprocal phase shift resulting from an induced frequency difference between counterpropagating beams in a fiber-optic coil to null out nonreciprocal phase shifts due to rotation. Thus it has the potential for wide dynamic range, high sensitivity and linear rotation sensing. It also has an inherently digital output desirable for modern guidance systems.
In improved forms passive fiber-optic gyros include a light source which has a wide spectral bandwidth and low coherence to avoid excess noise due to scattering. A central beamsplitter acts on the beam from the light source to generate two counterpropagating beams which are coupled into a fiber-optic coil. The counterpropagating beams exit the fiber-optic coil and are recombined by the beamsplitter. Upon rotation there is a fringe shift between the recombined beams given by Equation (1) ##EQU1## where R is the radius of the fiber-optic coil, .lambda. is the wavelength, .OMEGA. is the rotation rate, L is the length of the fiber-optic coil, and c is the speed of light. The recombined beams are reflected onto a detector that monitors the fringe shift through cosinusoidal intensity changes due to the rotation. Although this device is simple, it has nonlinear analog output, limited dynamic range, and is subject to errors due to intensity fluctuations of the output of the system.
To circumvent these problems, a nonreciprocal phase shift can be introduced into the system that nulls out phase shifts due to rotation. If the light from the source is emitted at frequency F.sub.o, and is split into counterpropagating beams by the central beamsplitter, the clockwise circulating beam of light passes through the fiber-optic coil at frequency F.sub.o while the counterclockwise beam circulates through the coil at frequency F.sub.o +F, where F is introduced into the counterclockwise beam by a suitable frequency shifter. The relative fringe shift caused by the frequency difference of F between the two beams propagating in the fiber-dptic coil is given by Equation (2) EQU Z.sub.F =-Ft.sub.D =-FLn/c (2)
where t.sub.D is the time delay through the fiber coil and n is its index of refraction.
In order for the system to be nulled, the fringe shift due to rotation must be off-set by the fringe shift due to the frequency difference of the light beams counterpropagating through the fiber-optic coil. That is, the criterion for a nulled condition is stated by Equation (3) EQU Z.sub.R +Z.sub.F =O (or any integer value, when using an offset frequency) (3)
Combining Equations (1) and (2), results in Equation (4). EQU F=2.OMEGA.R/.lambda.n (4)
To assure that the nulling condition of Equation (3) holds, an AC phase-sensitive detection scheme is included. Nonreciprocal phase shifts between the counterpropagating beams are introduced at a rate .omega.. When the condition of Equation (3) holds, second and higher order even harmonic signals appear on the detector. Upon rotation of the system, a first harmonic signal of .omega. falls onto the detector with an amplitude and phase dependent upon rotation rate along with higher order odd harmonics. This first harmonic signal is synchronously demodulated, and the resultant output voltage applied to an integrator, which in turn corrects the output frequency of a voltage-controlled oscillator, closing the feedback loop and nulling the system.
The performance of the gyro can be adversely affected by numerous problems such as drift which heretofore have prevented its use as an inertial grade gyro where bias drift due to wavelength shifts and nonreciprocal path lengths should be held to less than 0.1 part per million. When gyros are constructed with one frequency shifter, bias drift due to changing wavelength of the light source with age and temperature is a primary error factor. This can be corrected by employing designs using two frequency shifting modulators. With such designs, no satisfactory means are known to determine the scale factor, which depends on the instantaneous input light source frequency to approximately one part per million which is required to control an inertial grade gyro.